Unlike a conventional edge-emitting laser diode, a surface-emitting laser diode emits light in a direction orthogonal to a substrate, and a large number of devices can be arranged in a two-dimensional array on a single substrate, so the surface-emitting laser diode has received attention as a light source for digital copying machine or printer.
Conventionally, in a surface-emitting laser diode of this kind, a pair of multilayer reflecting mirrors are formed on a semiconductor substrate, and an active layer as a light emission region is included between the pair of multilayer reflecting mirrors. Then, a current confinement layer with a configuration having a narrowed current injection region is arranged on one of the multilayer reflecting mirrors so as to increase current injection efficiency into the active layer and to reduce a threshold current. Moreover, an n-side electrode and a p-side electrode are arranged on a bottom surface and a top surface, respectively, and a light emission opening for emitting laser light is arranged in the p-side electrode. In the surface-emitting laser diode, after a current is confined by the current confinement layer, the current is injected into the active layer, and light is emitted in the active layer, and while the light is repeatedly reflected by the pair of multilayer reflecting mirrors, the light is emitted from the light emission opening of the p-side electrode as laser light.
By the way, the above-described surface-emitting laser diode typically has such nonuniformity that the polarization direction varies due to device variations, or such instability that the polarization direction is changed by output or environmental temperature. Therefore, in the case where such a surface-emitting laser diode is applied to a polarization-dependent optical device such as a mirror or a beam splitter, for example, in the case where the surface-emitting laser diode is used as a light source for digital copying machine or printer, there is an issue that variations in the polarization direction cause a difference in imaging position or output, thereby blur or color unevenness occurs.
Therefore, to overcome such an issue, some techniques of stabilizing a polarization direction in one direction by arranging a polarization controllability function in the surface-emitting laser diode have been reported.
For example, as one of such techniques, there is a technique using a special inclined substrate which has a (311) plane as a normal and is made of gallium-arsenic (GaAs). In the case where a surface-emitting laser diode is formed through the use of such a special inclined substrate, gain characteristics with respect to the [−233] direction are enhanced, and the polarization direction of laser light is controllable to this direction. In addition, the polarization ratio of laser light is very high, so this technique is effective to stabilize the polarization direction of the surface-emitting laser diode in one direction.
Moreover, in Patent Document 1, a technique of controlling polarization by reducing the size of the section of a post structure to be smaller than the mode size of light is disclosed.
Moreover, in Patent Document 2, a technique of forming a discontinuity in a part of a metallic contact layer which does not have an influence on characteristics of laser light emitted from a light emission opening so as to obtain polarization in a direction parallel to a boundary of the discontinuity is disclosed.    Patent Document 1: Japanese Patent No. 2891133    Patent Document 2: Published Japanese Translation No. 2001-525995 of PCT international application